1. Field of the Invention
The present invention relates to an inertia force sensor that detects an inertia force, particularly an oscillation type inertia force sensor that detects the inertia force using an oscillator.
2. Description of the Related Art
An oscillation type inertia force sensor is used as, for example, an angular velocity sensor detecting the angular velocity based on inertia force. An oscillation type inertia force sensor includes an oscillator for detecting angular velocity, an oscillation circuit unit supplying a driving signal to the oscillator, and a detection circuit unit detecting the angular velocity of the oscillator. An oscillator includes an electrostatic driving and capacitance detection type, a piezoelectric driving and detection type, or the like. An oscillator includes an oscillating body that oscillates by the angular velocity, driving means for driving the oscillating body, monitor means for feeding back to an oscillation circuit unit a monitor signal according to the level of amplitude of the oscillating body (oscillating state of the oscillator), and detection means for providing a detection signal based on the oscillatory displacement caused by a Coriolis force of the oscillating body.
The oscillation circuit unit is formed as a self oscillation circuit of a closed loop with the oscillator as the resonant element to generate a driving signal from a monitor signal according to the level of amplitude of the oscillating body and supplies the driving signal to the oscillator to control the drive of the oscillating body. The detection circuit unit generates and outputs an angular velocity detection signal based on the detection signal applied from the detection means of the oscillator. The angular velocity detection signal corresponds to a DC voltage in accordance with the degree of the angular velocity of the oscillator.
An oscillation type inertia force sensor cannot detect the angular velocity until the amplitude of the oscillating body attains a predetermined level after the power is turned on. It is desirable to shorten the startup time starting from power activation and up to the amplitude of the oscillating body attaining a predetermined level at the oscillation type inertia force sensor. For example, Japanese Patent Laying-Open No. 11-281368 discloses an oscillation type inertia force sensor that can shorten the startup time. The oscillation type inertia force sensor disclosed in Japanese Patent Laying-Open No. 11-281368 includes a voltage amplifier inside or at a succeeding stage of a variable gain amplification circuit (VGA circuit) constituting an automatic gain control circuit (AGC circuit). Further, the voltage amplifier includes switching means for switching the amplification factor (gain) according to the voltage level of a monitor signal.
In a conventional oscillation type inertia force sensor, the monitor signal is amplified by a VGA circuit until the amplitude of the oscillating body attains a predetermined level after the power is turned on, attaining a state allowing detection of the angular velocity of the oscillator. Therefore, the startup time of the oscillation type inertia force sensor can be shortened by increasing the amplification factor of the VGA circuit.
For the purpose of reducing the startup time at the oscillation type inertia force sensor, increasing the amplification factor of the VGA circuit (to have a wide gain variable range) is required, leading to the necessity of a complicated circuit design. Even if the amplification factor of the VGA circuit could be increased by a complicated circuit design, it was difficult to amplify the monitor signal by the VGA circuit over the entire gain variable range to stabilize the operation of the automatic gain control circuit. Thus, a conventional oscillation type inertia force sensor had the problem that the startup time can be reduced only in a gain variable range inherent to the VGA circuit according to the amplification factor of the VGA circuit.